The present invention relates to thermal array imaging, printing or recording devices and is more particularly directed to a protection method and apparatus to prevent overheating of the thermal array device.
It is known in the art to fabricate thermal recording devices having imaging stylii arranged in a linear array. Such devices typically are comprised of a plurality of stylii which are formed by disposing electrically resistive material on an insulating substrate to form a plurality of individual stylii in a single row. These stylii are electrically connected to driver circuits. Each stylus is selectively energized by the driver circuits to produce Joule heat. When the stylii are brought into contact or suitable proximity with a thermally sensitive imaging medium, each energized stylus makes a mark on the medium. The stylii typically are spaced at a density of 100 stylii per inch and may require as much as one watt of power to raise the temperature of a single stylus to a level suitable for imaging. Energizing the stylii at a high repetition rate can cause overheating or even burn out of the stylii. Overheating of the stylii can also cause smudging or shadows on the recording medium.
To avoid an occurrence of overheating in thermal array stylii, the prior art teaches the use of various types of temperature compensating circuits. One such circuit, disclosed in U.S. Pat. No. 3,577,137 to James Brennan, Jr., uses a temperature sensor to sense the temperature of the stylii. The power applied to the stylii then is adjusted in order to reduce the heat. Such circuits require calibration and are therefore expensive to build and maintain and can also be subject to reliability problems.
Another method taught by the prior art to prevent overheating of a thermal array stylii is to control the "on" time of the incoming print command. U.S. Pat. No. 4,070,587 to Takayoshi Hanakata discloses a circuit using a "one shot" control principle, so that the drive current to the thermal stylus is cut off by the "one shot" after a predetermined interval. Such circuits, however, will not protect against overheating of the thermal stylii caused by rapid repetition of the stylus drive current.
Still another way to prevent overheating is accomplished in the prior art by the use of large metal heatsinks and by air cooling. Such devices add weight to the device and are not very efficient.